1. Field of the Invention
The invention relates generally to the field of seismic data processing. More particularly, the invention relates to methods for demigrating previously time-migrated seismic data so that post-stack depth migration processes can be used.
2. Background Art
Seismic surveying is used to evaluate structures of, compositions of, and fluid content of subsurface earth formations. A particular application for seismic surveying is to infer the presence of useful materials, such as petroleum, in the subsurface earth formations. Generally, seismic surveying includes deploying an array of seismic sensors at or near the earth's surface, and deploying a seismic energy source near the sensors also at or near the surface. The seismic energy source is actuated and seismic energy emanates from the source, traveling generally downwardly through the subsurface until it reaches one or more acoustic impedance boundaries in the subsurface. Seismic waves are reflected from the one or more impedance boundaries, whereupon it then travels upwardly until being detected by one or more of the seismic sensors. Structure and stratigraphic composition of the Earth's subsurface is inferred from, among other properties of the detected energy, the travel time of the seismic wave, and the amplitude and phase of the various frequency components of the seismic wave with respect to the energy emanating from the seismic source.
In order to infer the structures of subsurface earth formations from seismic waves measured at the earth's surface from the source/receiver position at the surface, it is necessary to determine the velocity of the various formations through which the seismic wave passes. Velocities of the earth formations can vary both with respect to depth in the earth (vertically), and with respect to geographic position (laterally). Seismic data, however, are recorded only with respect to time. Methods known in the art for estimating velocities of the earth formations both vertically and laterally rely on inferences about the travel path geometry of the seismic wave as it travels from the source to the various receivers deployed at the earth's surface.
In order for images produced from seismic data to correspond accurately to the spatial distribution of subsurface structures and compositional changes in the Earth's subsurface, techniques known generally as “time migration” and “depth migration” are performed on the seismic data. Migration is a process by which reflection events in seismic data are made to correspond in time (time migration) to the reflection times that would occur if seismic data acquisition geometry were identical for every surface position for which an image is produced, and in the case of depth migration, to have such events be located at the depths in the Earth at which they are located. Thus, migration is performed in two general classes of migration process. Time migration is used to cause the reflective events to be poisoned at the correct time in the image. Depth migration is used to cause the reflective events to be positioned at the correct depth in the image. Migration techniques are performed either “pre-stack” or “post-stack.” Post stack migration refers to migration techniques that are performed on seismic data for which numbers of individual data recordings (“traces”) are processed and summed to improve seismic signal to noise ratio. Pre-stack migration, by contrast, is performed on individual data recordings. Pre-stack migration typically produces better images. An effective method of pre-stack time migration is disclosed, for example, in Sun, C., Martinez, R., Amplitude preserving 3D pre-stack Kirchhoff time migration for V(z) and VTI media, 72nd Annual International Meeting, Society of Exploration Geophysicists, Expanded Abstracts, pp. 1224-1227 (2002).
Pre-stack depth migration typically produces the best image images comparing to the other type of migration. Pre-stack depth migration, however, is computationally intensive, and therefore relatively expensive, as compared with post-stack depth migration techniques. Pre-stack time migration techniques, such as the technique disclosed in the Sun et al. paper referred to above, are relatively computationally economical. What is needed is a technique to produce a stacked seismic section having the image quality of pre-stack depth migration while incurring pre-stack time migration computation cost.